An Inhibitory Factor associated with the Mammary Tumour Agent in Mice

ACTIVITY of the mammary tumour agent from mice has been associated with the supernatant fraction of extracts of stomach milk after ultracentrifugation 1 . In skimmed breast milk, containing the agent, considerable activity was found in pellets of ultracentrifuged milk 2,3 . Because activity is found in both supernatants and in pellets, there is an indication that there may be several factors causing mammary adenocarcinoma or that a fragment of a sedimentable agent is active. An attempt was made to isolate the active principle from the pellets which contained virus-like B particles, said to be the causative agent of mammary tumours 4,5 .